Propylene copolymers and terpolymers have been used in film applications because, with respect to propylene homopolymers, are characterized by a better impact, lower rigidity and better transparency. It is however difficult to find an acceptable balance between those properties that are often contrasting. For example, when a certain softness is desired, that is commonly obtained in the presence of high amount of xylene-soluble fractions that make them unsuitable for food contact applications. Moreover, for heat-seal applications it is important to have a sufficiently wide processability window, i.e. the range of temperatures comprised between the melting point and the seal initiation temperature.
International application WO 2009/019169 describes a process for producing terpolymers of propylene, ethylene and other alpha-olefins in a gas-phase reactor comprising two interconnected polymerization zones. The process is carried out in the presence of conventional Ziegler-Natta catalysts comprising a phthalate-based electron donor. The obtained terpolymers show a quite good combination of properties. There still exists the need to obtain propylene terpolymers endowed with further improved properties, particularly with respect to their low seal initiation temperature with as little as possible comonomer amount coupled with a sufficiently wide processability window. It has been found that those and other results can be achieved by using a specific class of Ziegler-Natta catalysts in the preparation of propylene terpolymers.